As one of the fuel economy improvement measures of an automobile triggered by global environment problems, weight reduction of the vehicle body is advancing, and it is necessary to high-strengthen a steel sheet used for an automobile as much as possible. On the other hand, when a steel sheet is high-strengthened, shape accuracy in press forming comes to deteriorate.
On this account, a hot press forming method has been employed for manufacturing components in which a steel sheet is heated to a predetermined temperature (for example, a temperature at which a state of an austenitic phase is achieved), the strength is lowered, the steel sheet is thereafter formed using a tool of a temperature (room temperature for example) lower than the steel sheet, thereby impartation of a shape and rapid heat treatment (quenching) utilizing the temperature difference of the both are executed simultaneously, and the strength after forming is secured. Also, such a hot-press forming method is referred to by various names such as a hot forming method, hot stamping method, hot stamp method, die quench method, and the like in addition to the hot press method.
FIG. 1 is a schematic explanatory drawing showing a tool configuration for executing hot press forming described above, 1 in the drawing is a punch, 2 is a die, 3 is a blank holder, 4 is a steel sheet (blank), BHF is a blank holding force, rp is punch shoulder radius, rd is die shoulder radius, and CL is punch/die clearance respectively. Also, out of these components, in the punch 1 and the die 2, passages 1a, 2a through which a cooling medium (water for example) can pass are formed inside of each, and it is configured that these members are cooled by making the cooling medium pass through these passages.
In hot press forming (hot deep drawing for example) using such a tool, forming is started in a state the steel sheet (blank) 4 is heated to a single-phase zone temperature of Ac3 transformation point or above and is softened. That is, in a state the steel sheet 4 in a high temperature state is sandwiched between the die 2 and the blank holder 3, the steel sheet 4 is pressed in to the inside of a hole of the die 2 by the punch 1, and is formed into a shape corresponding to the shape of the outer shape of the punch 1 while reducing the outside diameter of the steel sheet 4. Also, by cooling the punch 1 and the die 2 in parallel with forming, heat removal from the steel sheet 4 to the tools (the punch 1 and the die 2) is executed, holding and cooling are further executed at a forming bottom dead point (the temporal point the tip of the punch is positioned at the deepest point: the state shown in FIG. 1), and thereby quenching of the raw material is executed. By executing such a forming method, a formed product of 1,500 MPa class with excellent dimensional accuracy can be obtained, the forming load can be reduced compared with a case a component of a same strength class is cold-formed, and therefore less capacity of the press machine is needed.
As a steel sheet for hot pressing use widely used at present, one using 22Mn-B5 steel as a raw material is known. The steel sheet has the tensile strength of approximately 1,500 MPa and the elongation of approximately 6-8%, and is applied to a shock resistant member (a member not causing deformation as much as possible and not causing breakage in collision). However, application to a component requiring deformation such as an energy absorption member is difficult because elongation (ductility) is low.
As a steel sheet for hot pressing use exerting excellent elongation, technologies such as the patent literatures 1-4 for example have also been proposed. According to these technologies, the basic strength class of each steel sheet is adjusted by setting the carbon content in the steel sheet to various ranges, and elongation is improved by introducing ferrite with high deformability and reducing the average grain size of ferrite and martensite.
On the other hand, although automobile components are required to be joined by spot welding, it is known that, in the hot stamp formed product whose microstructure is mainly of martensite, drop of the strength in the weld heat affected zone (HAZ) is extreme, and the strength of the welded joint drops (is softened) (non-patent literature 1 for example).